Crystal pulling apparatus



Filed Oct. 10, 1963 INVENTOR AL BRIG/ 7' a. F/SCHE/i a M m s V. N M T T A United States Patent "i e Patented j 3,268,297 CRYSTAL PULLING APPARATUS Albrecht G. Fischer, Trenton, N.J., assigncr to the United States of America as represented by the Secretary of the Air Force Filed Oct. 10, 1%3, Ser. No. 315,415 6 Claims. (Cl. 23-273) This invention relates generally to Czochralski type pullers for pulling crystals from a melt containing ma terials which are utilized in crystal production for the semiconductor, electrical and optical fields. More particularly, the invention relates to apparatus which is suitable for use with materials where the minimum temperature to maintain the vapor pressure in the apparatus lies above the Curie point of magnetic alloys.

Previously utilized devices for performing this function have been developed to the type described in the patent to R. Gremmelmaier, No. 3,074,785, granted on January 22, 1963. This apparatus utilizes a sealed vessel having one temperature applied to the melt and another to the environment Within the sealed vessel. The device encompasses the utilization of substances which may include alloys having at least one component element whose partial vapor pressure above the melt is greater than the partial pressure of the other components and any resultant compound of the processed components. The Gremmelmaier apparatus provides a movable pulling plunger within the sealed melting vessel with operation of the plunger being effected by means of an external magnetic system. A soft iron core is provided in the sealed vessel and is secured to the plunger.

When ZnSe, ZnTe, ZnS, CdS, CdSe or CdTe are utilized with the Gremmelmaier sealed system, the apparatus will not perform the desired function since these compounds can not be crystallized unless the temperature that is needed to maintain the necessary vapor pressure within the ampoule is higher than the Curie point of the magnetic core. The maximum tolerable core temperature is approximately 700 C.; however, temperatures above this are required to prevent decomposition of the aforementioned compounds. Since no magnetic force can be transferred through a quartz Wall to suspend the core if the core is not magnetic, the apparatus previously described is not operable.

Instead of lifting and rotating the seed as provided by Gremmelmaier, the present invention eliminates the presentation of the magnetic core in the vapor space above the melt and, instead, uses a rotating crucible, which is held in place vertically while the ampoule is made to rise. The crucible is held by magnetic cores which are immersed in a bath of the liquid which constitutes the vapor atmosphere. Since this liquid is heated only at its uppermost portion to the necessary temperature to give the pressurized, saturated atmosphere, the lower part of the liquid is kept much cooler. At these lower temperatures, which would be just above the freezing point, for example of liquid zinc at 410 C., the magnetic susceptibility of steel is undamaged and magnetic forces can be transferred.

Accordingly, it is a primary object of this invention to provide a Czochralski type pulling apparatus for compounds requiring pressurized vapor atmospheres with minimum temperatures up to 1100 C., or the limit of the strength of quartz glass.

It is another object of this invention to provide a magnetic Czochralski type crystal puller for compounds wherein the temperature of the vapor is equal to or greater than the Curie temperature of a magnetic core.

It is still another object of this invention to provide a Czochralski type crystal puller with improved reliability and operability.

It is a further object of this invention to provide a Czochralski type crystal puller wherein the melt for creating the vapor atmosphere is heated only at its top to generate the vapor pressure, whereas the bulk is maintained at a much lower temperature.

It is a still further object of this invention to provide a magnetic system for creating the relative movement between the seed crystal and the melt wherein the iron cores utilized in the magnetic system are maintained at a temperature below the Curie point, even though the vapor temperature required for the proper vapor pressure is greater than the Curie point.

Another object of this invention involves the provision of a novel Czochralski type magnetic crystal puller which utilizes conventional, currently available materials that lend themselves to standard mass production manufacturing techniques.

These and other advantages, features and objects of the invention will become more apparent from the following description taken in connection with the illustrative embodiment in the accompanying drawing, wherein the figure is a schematic representation of the magnetic crystal puller of this invention.

Referring to the figure, there is shown a quartz ampoule 10 which is adapted for vertical movement while the crucible therein is maintained at a constant vertical level. Vertical movement is achieved by means of the vertical support 11 which is secured to the top and bottom portion of the ampoule 10 at 12. At the bottom of the top section 13 of the quartz ampoule 10 there is attached a seed holder 14 from which is suspended the crystal 30 which is to be pulled from the melt. The ampoule 10 is basically cylindrical; however, in order to insert the necessary structure within the unit, the top section 13 is formed separately and then secured in a conventional manner with the cylindrical portion of the ampoule, as shown. Once the unit is assembled, the device is sealed and the ampoule, for all practical purposes, becomes a unitary structure.

A drive mechanism for Vertical movement is shown generally at 16 and comprises an internally threaded block 18 with a threaded member 20 extending therein. If the member 20 is maintained fixed in a horizontal plane, rotation of the member 20 will cause vertical movement of the support 11 and, therefore, vertical movement of the ampoule It? and seed holder 14.

Assuming the apparatus is designed for use with ZnTe, a thermocouple 22 is provided in the interior of the hollow top portion 13, which is formed of quartz and has contained thereinquartz wool 24 which provides a seat for the thermocouple 22. A projection 26 on the bottom of top ampoule portion 13 provides the thermocouple 22 with access to the volume within the ampoule 10. The thermocouple is arranged in a conventional manner to control a heater 28, which for zinc would maintain a temperature of about 960 C. which is sufficient for the maintenance of zinc Vapor at approximately 1.5 atmospheres.

The seed crystal 30 is maintained within the holder 14 and is made to extend within a melt of, for example, ZnTe. The melt is contained within a crucible 32, for example, of graphite which is maintained at a temperature of 1280 C. by means .otf a conventional RF coil '34 which surrounds the quartz ampoule 10, which in turn houses the apparatus. The temperature above the melt is maintained at about 0 C. by means of transparent heater 36. A support 38 is attached to the crucible 32 and is connected with a rotor 40 which may be of quartz which contains iron cores 4-2. The zinc vapor is provided by liquid zinc at '44 which is arranged to surround the rotor 40. The Zn is heated to a temperature lower than that of the crucible, for example, at approximately 950 C. A heater 46, surrounding the ampoule 10, maintains the top of the volume of Zn at the stated temperature, which is suflicien-t to provide the proper Zn vapor atmosphere. Control of this heater is effected in a conventional manner by a thermocouple 45.

The rotor 40 to which the crucible 32 and crucible holder 38 are secured rotates within the ampoule '10, which is free to ride up and down on the rotor in a vertical direction under the action of the vertical drive 16. The rotor 40 comprises an extension of the holder 38 which is perforated and hollow to allow passage of the Zn. A heater 47, is controlled by a thermocouple 49 inserted in a pump stem 48 on the ampoule. A bearing surface at 80 can be provided on the interior of the hollow portion in order to provide guidance for the rotor. In addition, this portion, which is formed as a part of the bottom 82 of ampoule 10, houses the thermocouple 45 and heater '47. The heater 47 maintains liquid zinc at 44 at approximately 550 C. around the rotor 40. Insulated from the rotor 40 in a hollow portion concentric with the central axis of the rotor are iron cores 42. The insulation is provided by Th at 41 in order to protect the cores from the higher temperature of the zinc immediately above the rotor. A break tip is provided at 60 to drain the zinc from the rotor when desired.

A support 62 is provided for the spin drive generally indicated at 64. The spin drive 64 comprises a pair of retating magnets 66 which rotate in the direction of the arrows. The magnets are secured to a plate 68 which is antimagnetic and which in turn is supported by ball bearings 70 on the support 62. A gear unit at 72 provided to cause rotation of the plate 68 and the magnets 66, which cause rotation of the iron cores 42 of the rotor 40, the holder 38 and, therefore, the crucible 32. Vertical movement, provided by the unit 16, in conjunction with the magnetic rotation provides the proper pulling and rotation for Czochralski type operation While the rotor remains vertically fixed by the magnets 66.

Even though the temperatures within the unit are above the Curie point of the magnetic cores 42, separation is ettected such that the magnetic force's are able to be utilized where the vapor temperature is above the Curie temperature.

From the foregoing, it may be seen that a vertical drive is provided tor an ampoule to which a seed holder and seed are attached. The movement of the ampoule is vertical relative to that of a crucible which is rendered movable only in rotation by means of a rotating magnet system. Thus, the Czochralski type action is achieved. In addition, heaters 28, 46 and 4-7, together with the RF coil 34, provide the proper temperatures within the ampoule and rotor such that the space above the rotor is mainvapor atmosphere. The molten element surrounding and above the rotor is maintained in a liquid state such that the su-r-iiace presented to the volume within the ampoule above the rotor is maintained at a much higher temperature than that which is adjacent the iron cores. This arrangement allows for the utilization of compounds which require vapor temperatures much greater than the Curie temperature of the magnet-izahle material required for the rotating drive. Of course, the materials utilized in the construction of the turnace portion of the apparatus would be of quartz or graphite as is conventional in the art.

Although the invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that the invention is oapalble of a variety of alternative embodiments within the spirit and scope of the appended claims.

I claim:

1. An apparatus tor pulling crystals from a melt of a material in a gaseous atmosphere requiring temperatures above the Curie point of magnetizable materials comprising, a closed ampou'le, a seed holder fixedly secured to the interior of one end of said ampoule, means secured to said am-poule for causing vertical movement of it and said seed holder, a rotor located within said arnpoule and below said seed holder, a crucible secured to said rotor and extending above said rotor to a position adjacent said seed holder, magnetizable means at the lower end of said rotor, a melt of material in the lower portion of the ampoule surrounding the magnetizable means to provide the said gaseous atmosphere, and rotating magnet means proximate to said magnetizable means and mounted externally around said ampoule for rotation to cause spinning of said rotor and crucible While maintaining them in a horizontal plane.

2. An apparatus as defined in claim 1 including means connected with said rotating magnets for causing rotation thereof.

3. An apparatus as defined in claim 1 including insulating means within said rotor and surrounding said magnetizable means.

4. An apparatus as defined in claim 1 including means for providing temperatures at the upper surface of said melt and above said melt temperatures greater than 700 C.

5. An apparatus as defined in claim 4 including means for maintaining the portion of said melt below the surface thereof at a temperature of below 700 C.

6. An apparatus 'for pulling crystals from a melt of a material in a gaseous atmosphere requiring temperatures above the Curie point of magnetizable materials comprising, a closed arn-poule, a seed holder secured to the interior of one end of said ampoule, means secured to said ampoule hor causing vertical movement of it and said seed holder, a hollow rotor located within said ampoule and below said seed holder, a crucible secured to said rotor and extending above said rotor to a position adjacent said seed holder, magnetizalble means within said rotor, rotating magnet means in a common plane with the lower extremity of said rotor proximate to said magnetiazle means and mounted externally around said ampoule for rotation to cause spinning of said rotor and crucible while maintaining them in a horizontal plane, a bearing surface projecting into the interior of the hollow of said rotor, a heater mounted within said bearing surface, the rotor and bearing surface being immersed in the anion of the compound being crystallized, said heater position within said hearing being arranged to heat the lower portion of the anionic fluid, and a second heater in encircling relationship to the ampoule disposed so as to heat the upper portion of the anionic material.

References Cited by the Examiner UNITED STATES PATENTS 2,686,864 8/1954 Wroughton et al. 3,057,703 10/1962 Knapic 23273 3,074,785 1/1963 Gremmelmaier 23273 3,088,853 5/1963 Harper 23273 FOREIGN PATENTS 629,412 10/ 1961 Canada.

NORMAN Y-UDKOFF, Primary Examiner.

G. HINES, Assistant Examiner. 

1. AN APPARATUS FOR PULLING CRYSTALS FROM A MELT OF A MATERIAL IN A GASEOUS ATMOSPHERE REQUIRING TEMPERATURES ABOVE THE CURIE POINT OF MAGNETIZABLE MATERIALS COMPRISING, A CLOSED AMPOULE, A SEED HOLDER FIXEDLY SECURED TO THE INTERIOR OF ONE END OF SAID AMPOULE, MEANS SECURED TO SAID AMPOULE, FOR CAUSING RENTICAL MOVEMENT OF IT AND SAID SEED HOLDER, A ROTOR LOCATED WITH SAID AMPOULE AND BELOW SAID SEED HOLDER, A CRUCIBLE SECURED TO SAID ROTOR AND EXTENDING ABOVE SAID ROTOR TO A POSITION ADJACENT SAID SEED HOLDER, MAGNETIZABLE MEANS AT THE LOWER END OF SAID ROTOR, A MELT OF MATERIAL IN THE LOWER PORTION OF THE AMPOULE SURROUNDING THE MAGNETIZABLE MEANS TO PROVIDE THE SAID GASEOUS ATMOSPHERE, AND ROTATING MAGNET MEANS PROXIMATE TO SAID MAGNETIZABLE MEANS AND MOUNTED EXTERNALLY AROUND SAID AMPOULE FOR ROTATION TO CAUSE SPINNING OF SAID ROTOR AND CRUCIBLE WHILE MAINTAINING THEM IN A HORIZONTAL PLANE. 